The Princeton grad accused of executing his hedge fund manager father after he threatened to cut the 30-year-old's allowance may have burned down a friend's historic mansion in a jealous rage in September.

The New York Post reports that Tommy Gilbert Jr. is the prime suspect in an arson that completely destroyed a 19th century Hamptons home belonging to the father of Peter Smith Jr., a former friend.

The Post quotes anonymous sources as saying that Gilbert Jr. retaliated because he believed that Smith Jr. was sleeping with his girlfriend.

The Post also claims that Gilbert Jr. killed the Smith family dog, which was found dead in their driveway. However, sources close to the events denied this to Daily Mail Online.

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Former flame: Anna Rothschild recently dated Tommy Gilbert Jr, the 30-year-old man accused of murdering his financier father over a weekly allowance

Broken up: The couple met at a mutual friend's party in December 2013. Pictured above last year at an event at The Jane Hotel. Miss Rothschild says they broke up last May after she started dating someone new

Gruesome: Above, investigators remove Thomas Gilbert Sr's body from his apartment building on Manhattan's Upper East Side

Just three days after the fire in Sagaponack, New York, Gilbert Jr. was arrested for violating a protective order taken out by Smith Jr.

Gilbert Jr. was arraigned on Monday on murder and weapons possession charges in the death of his father, 70-year-old Thomas Gilbert Sr., who was found shot in the head in his Manhattan apartment on Sunday.

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Gilbert Jr.'s ex, meanwhile, insisted in an interview with ABC News that he is a 'sweet man' who is tortured by his relationship with his demanding father.

Twice-divorced Upper East Side socialite Anna Rothschild, 49, dated Gilbert Jr. for several months until he broke it off in May when Gilbert Jr. met someone new.

Miss Rothschild, 49, says she was shocked and surprised to hear her ex-boyfriend was implicated in his father's death, since Tommy seemed happy and 'totally fine' when she saw him at a dinner just two weeks ago.

Pattern of violence? Tommy was arrested and accused in September of harassing a Peter Smith Jr. in Southampton, Long Island. A law enforcement official told the Associated Press that Tommy was suspected of burning down Smith's father's mansion there three days before his arrest

Love rival? Sources told the New York Post that Gilbert Jr. set fire to the family home of Peter Smith Jr. (left) because he believed Smith was sleeping with his girlfriend. Smith's sister, Bettina Prentice, and brother Chris Smith are pictured at right

Party boy: Gilbert Jr was a fixture on the New York social scene with a host of beautiful women ready to pose for pictures with him including Arielle Patrick (left) and Maddie Haddon (right)

Society friends: Alex Brice, Amy Poliakoff and Tommy Gilbert pose together at Greensward Circle's 7th Annual 'Evening in the Garden' in June 2012

Social: Tommy Gilbert, pictured with friend Monika Plocienniczak, at the Young New Yorkers Summer Party in the Hamptons

However, she she was frank when she described his 'strained' and 'stressful' relationship with his father.

'His father was never happy with anything he did or nothing was ever good enough,' she said.

Miss Rothschild first met Tommy at a mutual friend's dinner party in December 2013, and soon after started dating. She says they broke up last May when she started dating someone new.

At the time, Miss Rothschild says Tommy was unemployed but working to start his own hedge fund, just like his father.

In a separate interview with the New York Post, Miss Rothschild revealed that Tommy complained about his father not giving him the seed money to start the investment venture.

Still, money never seemed to be too much of an issue during their courtship since Tommy always paid the bill when they went out and seemed to enjoy his life of leisure.

Miss Rothschild says Tommy appeared not to be too bothered about his lack of a job, spending most of his time in the Hamptons, going to the gym, practicing yoga and surfing.

She describes Tommy as sweet but a bit of an introverted loner who had few friends.

In fact, Tommy's mother was about the only person who ever called him, Miss Rothschild says.

Still, she finds it hard to believe her former flame capable of carrying out such a heinous act.

'How could a guy be that gorgeous, that wealthy, that fit and kill his dad? This is the last thing in a million years that I thought he could do,' Miss Rothschild said.

Tommy Gilbert was part of the trust-fund-baby crowd, a handsome Princeton grad who flitted between his Manhattan home and the Hamptons and attended society parties at museums and symphonies while living off an allowance from his parents.

Behind bars: Tommy seen being escorted into Manhattan Central booking by a police officer

No bail: The 30-year-old was arraigned Monday on murder charges and jailed without bail. His attorney had no comment

On the surface, it was a charmed life.

But he was in debt, with no job or recent work history. He was a suspect in a fire that destroyed a mansion on Long Island, a law enforcement official told the Associated Press. He was accused of harassing a friend. And his mother told police he had a tense relationship with his Wall Street father and a history of mental illness, authorities said.

On Sunday afternoon, police say, it all erupted in violence over a small amount of money.

Threatened with a cut of a few hundred dollars in his monthly allowance, Gilbert went to his parents' apartment on wealthy Beekman Place on Manhattan's East Side and shot his father to death, authorities said.

The 30-year-old was arraigned Monday on murder charges and jailed without bail. His attorney had no comment.

Gilbert had gone to his parents' home saying he wanted to have a word alone with his father, hedge fund founder Thomas Gilbert Sr. His mother went out to get him some food and was gone about 15 minutes when she got a "bad feeling" and went back, said Robert Boyce, chief of detectives.

She found her 70-year-old husband on the bedroom floor, a bullet hole in his head, Boyce said.

A .40-caliber Glock was resting on his chest, his left hand on the gun. Boyce said it was a staged to look like suicide, but the gun wasn't in the right position for a self-inflicted wound.

Detectives waited outside Gilbert Jr.'s $3,000-a-month apartment in a six-floor walkup in Chelsea. When they saw a light go on, they went in and found bullets and empty shell casings along with 21 blank credit cards and a device for stealing card numbers, police said.

"He was on a stipend from the family. Looks like he was in debt," Boyce said.

Gilbert was taken in for questioning but he refused to speak to detectives and asked for an attorney.

His father, a graduate of Princeton University and Harvard Business School, worked on Wall Street for more than 40 years and founded Wainscott Capital Partners Fund in 2011. The fund has $200 million in assets.

The shooting was a rare act of violence in the well-to-do neighborhood near the United Nations.

Gilbert Jr. graduated from Princeton in 2009 with a degree in economics and was trying to follow in his father's footsteps: He had filed papers with the Securities and Exchange Commission to start a hedge fund of his own, Mameluke Capital.

He attended galas and fundraisers with the city's wealthy. In many society photos, some taken at such places as the Frick Collection and the New York Philharmonic, he was seen posing with smiling young women and looking tan at the beach.

But he was arrested and accused in September of harassing Peter N. Smith in the town of Southampton, on Long Island, and had a Feb. 2 court date. And a law enforcement official, who was not authorized to discuss the investigation and spoke to The Associated Press on condition of anonymity, said Gilbert is a suspect in a blaze that burned down Smith's father's mansion three days before his arrest.